1. Field of the Invention
The present invention relates to an accumulator incorporating a flow damper which is capable of statically switching flow rates from large to small, and to a method of manufacturing the flow damper. The present invention is useful when applied to an emergency injection system for a reactor in a pressurized water reactor (PWR) power plant, for example.
2. Description of the Related Art
An emergency core cooling system is installed in the PWR power plant. The emergency core cooling system includes an accumulator and so forth on the assumption that the PWR might cause a loss of primary coolant accident.
Water (coolant) is stored in the accumulator, and the water stored therein is pressurized by a pressurizing gas (nitrogen gas) which is filled in an upper part in the accumulator. Moreover, a flow damper is provided in the accumulator. The flow damper can switch a water injection flow rate in a reactor from a large flow to a small flow statically (without moving any part thereof). The flow damper includes a vortex chamber, a large flow pipe, a small flow pipe, an outlet pipe and the like, and is disposed at the bottom in the accumulator (see FIG. 1). A tip end of the outlet pipe is connected to a low temperature pipeline of a reactor primary coolant loop with a check valve interposed in between. The check valve is used for avoiding a back flow from a rector primary cooling system to the accumulator.
If the pipeline or the like in the reactor primary cooling system of the PWR power plant is broken and the coolant flows out of a crack to the outside (i.e. upon occurrence of a loss of primary coolant accident), the amount of the coolant in a reactor vessel may be reduced, and thereby a reactor core may become exposed. In this situation, however, if a pressure of the primary cooling system drops below a pressure in the accumulator, the water stored in the accumulator is injected from the primary cooling system pipeline into the reactor vessel through the check valve, and thereby refloods the reactor core.
In this case, the reactor vessel is refilled quickly by injecting water at a large flow rate at an initial stage thereof. Then, it is necessary to switch the water injection flow rate from the large flow to a small flow at a later stage when the reactor core is reflooded, because excessively injected water may spill out of the crack. In order to ensure this water injection flow rate switching operation, a reliable flow damper without a moving part is used for the accumulator.
The principles of the water injection flow rate switching by use of such a flow damper will be explained on the basis of FIGS. 8A to 8C (horizontal sectional views).
As shown in FIGS. 8A to 8C, a flow damper 10 has a structure in which a large flow pipe 2 and a small flow pipe 3 are connected to a peripheral portion (a circumferential portion) of a cylindrical vortex chamber 1, while an outlet 4 is formed in the center of the vortex chamber 1. The large flow pipe 2 and the small flow pipe 3 extend in mutually different directions from the outlet 4. Specifically, the small flow pipe 3 extends in the left direction along a tangential direction to the peripheral portion (the circumferential portion) of the vortex chamber 1. Meanwhile, the large flow pipe 2 extends in the right direction while forming a predetermined angle θ with the small flow pipe 3. Moreover, although illustration is omitted, an inlet of the small flow pipe 3 is located at the same level as the vortex chamber 1. Meanwhile, the large flow pipe 2 is connected to a standpipe which extends upward. An inlet of this standpipe is located higher than the vortex chamber 1 and the inlet of the small flow pipe 3. Furthermore, an outlet pipe is connected to the outlet 4 of the vortex chamber 1.
Moreover, since the water level in the accumulator is higher than the inlet of the large flow pipe 2 at the initial stage of water injection, the water in the accumulator flows into the vortex chamber 1 from both of the large flow pipe 2 and the small flow pipe 3 as indicated with arrows A and B in FIG. 8A. As a result, the injected water (a jet) from the large flow pipe 2 collides with the injected water (a jet) from the small flow pipe 3, and angular momenta of the jets are offset. In this way, the water flows directly toward the outlet 4 as indicated with an arrow C in FIG. 8A. Specifically, no vortex is formed in the vortex chamber at this time. Accordingly, a flow resistance is reduced at this time, and thus a large amount of water flows out of the outlet 4 and is injected into the reactor vessel.
By contrast, at the later stage of water injection, the water level in the accumulator drops below the inlet of the standpipe connected to the large flow pipe 2. Accordingly, there is no water flow from the large flow pipe 2 into the vortex chamber 1, and the water flows into the vortex chamber 1 only through the small flow pipe 3 as indicated with an arrow B in FIG. 8B. As a result, the injected water from this small flow pipe 3 proceeds to the outlet 4 while forming a vortex (a swirling flow) as indicated with an arrow D in FIG. 8B. Accordingly, the flow resistance is increased by the centrifugal force at this time, and an outflow (the water injected to the reactor vessel) from the outlet 4 becomes a small flow. This device is called a flow damper because it has the function to damp the flow rate as described above.
As described above, the accumulator currently in development is the advanced accumulator which is capable of switching flows from large to small statically and securely by including the flow damper 10. Moreover, the flow damper 10 of this advanced accumulator is required to define a proportion between the large flow and the small flow as high as possible in order to achieve a reasonable tank volume. For this reason, it is essential not to form a vortex in the vortex chamber by surely offsetting the angular momenta between the jet from the large flow pipe 2 and the jet from the small flow pipe 3 at the time of the large flow injection. Meanwhile, it is necessary to generate a high flow resistance by forming a strong vortex in the vortex chamber 1 when switching from the large flow to the small flow.
Accordingly, at the time of the small flow injection, the strong vortex is formed in the vortex chamber 1 by connecting the small flow pipe 3 along the tangential direction to the peripheral portion (the circumferential portion) of the vortex chamber 1.
However, as shown in FIG. 8C, the water flow (a free jet) blown out of the small flow pipe 3 into the vortex chamber 1 not only includes a direct flow along the tangential direction (see a dashed line E) but also includes a flow which spreads out of the extension line E of an inner surface 3a, at the side of the large flow pipe 2, of the small flow pipe 3 as indicated with a dashed line F (a proportion of spread caused by this free flow is approximately equal to 1/10). By contrast, in a conventional structure of a wall facing the small flow pipe, a junction 6 of the large flow pipe 2 (an inner surface 2a) and the vortex chamber 1 (an inner surface 1a) is located either on the extension line E or inside the extension line E. For this reason, part of the jet from the small flow pipe 3 (free-jet-spreading portion) collides with the inner surface 2a of the large flow pipe 2 as indicated with an arrow G, and flows into the vortex chamber 2 while bypassing the junction 6 of the large flow pipe 2 and the vortex chamber 1.
As a result, the part of the jet is detached from the inner surface 1a of the vortex chamber 1, and flows in an inclined flow direction toward the center of the vortex chamber 1 as compared to the aforementioned tangential direction. Due to an influence of this flow, the overall flow direction of the jet from the small flow pipe 3 is inclined toward the center of the vortex chamber 1 as compared to the tangential direction (as indicated with an arrow B1). Accordingly, the vortex formed in the vortex chamber 1 is weakened by reduction of the angular momentum of the jet.